The present invention relates to an ultrasonic flow sensor and more particularly to a surgical system and cassette having an ultrasonic flow sensor.
Conventional ophthalmic surgical instrument systems use vacuum to aspirate the surgical site and positive pressure to irrigate the site. Typically, a cassette is serially connected between the means used to generate pressure and the surgical instrument. The use of cassettes with surgical instruments to help manage irrigation and aspiration flows at a surgical site is well known. U.S. Pat. Nos. 4,493,695 and 4,627,833 (Cook), U.S. Pat. No. 4,395,258 (Wang, et al.), U.S. Pat. No. 4,713,051 (Steppe, et al.), U.S. Pat. No. 4,798,850 (DeMeo, et al.), U.S. Pat. Nos. 4,758,238, 4,790,816 (Sundblom, et al.), and U.S. Pat. Nos. 5,267,956, 5,364,342 (Beuchat) and U.S. Pat. No. 5,747,824 (Jung, et al.) all disclose ophthalmic surgical cassettes with or without tubes, and they are incorporated in their entirety by this reference. Aspiration fluid flow rate, pump speed, vacuum level, irrigation fluid pressure, and irrigation fluid flow rate are some of the parameters that require precise control during ophthalmic surgery.
Prior art devices have used pressure sensors in the aspiration and irrigation lines and calculate fluid flow rates based on the sensed pressure. In the past, measuring of fluid pressures in surgical cassettes has been very precise and as the resistance in the fluid paths is known, fluid flow rates can be calculated reliably from fluid pressure. Recent improvements in the reliability of ultrasonic flow sensors, however, have now made it possible to non-invasively measure fluid flow accurately.
For example, one ultrasonic flow sensor disclosed in U.S. Pat. No. 6,098,466 (Shkarlet) discloses a flow sensor capable of accurately measuring fluid flow in vessels or tubes having decreased sensitivity to flow distribution non-uniformities and decreased overall size by employing multiple angled reflector surfaces which cause incident ultrasonic waves from one or more ultrasonic transducers to pass through the flow volume multiple times and in multiple directions without changing the planar orientation of the ultrasound waves. The wave paths resulting from the multiple reflections and multidirectional illumination of the flow volume decreases the probe's size and sensitivity to spatial distribution non-uniformities. The multiple angled reflector surfaces also permit the transmitting and receiving ultrasonic transducers to be placed close to one another, thereby reducing the overall probe size and making them particularly useful for incorporation in the relatively small fluid flow cassette used in ophthalmic surgery. In order for an ultrasonic flow sensor to work, the transducer must be acoustically coupled to the tubing in which the fluid is flowing so that any air located between the transducer and the tubing is removed. Prior art flow sensors generally use an acoustic gel, such as a high water content hydrogel material, to accomplish the acoustic coupling. When the acoustic coupling needs to be used in connection with a surgical cassette installed within a surgical console, sterility and cleanliness are of concern, making an acoustic gel less desirable than an acoustic coupling that is formed as part of the cassette or the console and that requires no gel.
Canadian Patent Application No. 2,127,039 A1 describes an elastomer for use as an acoustic coupler for ultrasonic devices. As described in this patent application, the difficulty with independently formed elastomeric acoustic couplers is providing intimate contact between the ultrasound transducer and the elastomer so that no air voids are present at the interface. The solution described in this patent application is an elastomer that is extremely soft and flexible and acoustically transparent. These properties allow the use of relatively thick couplers that may be easily compressed by the transducer, thereby providing greater and firmer contact between the transducer and the elastomer.
When used in connection with a surgical cassette installed within a surgical console, a preformed elastomeric acoustic coupler must be either attached to the cassette or to the ultrasound transducer located in the console. The use of an adhesive is undesirable because of the possibility of air bubbles at the interface of the elastomeric coupler and the surface to which it is adhered, and the fact that the adhesive may interfere with the transmission of the ultrasound waves. In addition, an adhesive adds additional interfaces in the acoustic path. Each additional interface degrades the acoustic signal and the sensing system reliability, repeatability and sensitivity.
Accordingly, a need continues to exist for a simple, reliable and accurate acoustic coupler that can be used on or with a surgical cassette.